In a wireless communication apparatus such as a cell phone, a power amplifier circuit device is usually provided at a transmission side for amplifying signals to be transmitted. The power amplifier circuit device includes semiconductor amplification elements such as metal-oxide-semiconductor field effect transistors (“MOSFETs”) formed as an integrated circuit (“IC”.)
A semiconductor IC, however, is generally susceptible to an electrostatic discharge (“ESD”) event, which may damage or destroy the IC. An ESD event is an electrical discharge of a current (positive or negative) for a short duration during which a large amount of current is provided to the IC. The high current may be built-up from a variety of sources, such as the human body and machines, referred to as the human body model (“HBM”) and machine model (“MM”), respectively. An IC is particularly susceptible to the HBM and MM built-up during fabrication, transportation, or handling.
Diodes have been generally used for ESD protection in ICs because they can be easily implemented with relatively simple manufacturing processes. However, an ESD protection circuit using diodes alone may not be suitable for protecting a power amplifier circuit device from ESD events, as is explained below by reference to FIG. 1. FIG. 1 is a schematic diagram of a system 10 using a conventional ESD circuit for protecting a power amplifier. System 10 includes an ESD protection circuit 12, a conductive pad 14, and a power amplifier 16. ESD protection circuit 12 includes a first diode 12-1 coupled between a first power line VDD and conductive pad 14, and a second diode 12-2 coupled between conductive pad 14 and a second power line VSS. Conductive pad 14 is connected to a matching circuit 16-1, for example, an RLC circuit, of power amplifier 16 for receiving an output voltage of power amplifier 16. Power amplifier 16 is connected between first power line VDD and second power line VSS. The voltage level of VDD line is approximately 2.5 volts (V). Power amplifier 16 may output a voltage of approximately 2 VDD or 5V to conductive pad 14 during normal operation. The output voltage of 5V, greater than VDD plus the cut-in voltage (for example, 0.7V) of first diode 12-1, turns on first diode 12-1, which should not become conductive during normal operation. The unexpected turn-on of first diode 12-1 may cause malfunction of power amplifier 16.
Other examples of ESD protection for power amplifier circuit devices can be found in U.S. Pat. No. 6,459,340 to Apel et al., entitled “Power Amplifier Mismatch Protection with Clamping Diodes in RF Feedback Circuit” (“the '340 patent”) and U.S. Pat. No. 6,624,999 to Johnson, entitled “Electrostatic Discharge Protection Using Inductors” (“the '999 patent”). The '340 patent discloses a circuit including a set of clamping diodes formed between terminals of a transistor acting as a power amplifier. The circuit may not quickly respond to an ESD event due to low conduction speed of the multiple diodes. The '999 patent discloses a circuit including an inductor coupled between an IC to be protected and an ESD clamping device. Since the inductor is formed external to the IC, the IC itself is not provided with ESD protection and may be exposed to an ESD event during transportation and handling.